Yup, you nailed it Leo, 11.5 and the scripts are shown in toolbars/scripts, and once I checked it, it becomes available as a command.
Thanks guys 
Anyone can be a beta tester these days
I'd like to report the solution I use today.
Use the JoinMP3 script by tbone with the proper button you can find here.
It works as a charm 
It's wise to check the mp3 produced by ffmpeg for errors and fix them. Just follow this step requiring only one click.